The present invention relates to an OFDM (Orthogonal Frequency Division Multiplex) receiver such as a digital terrestrial broadcasting receiver, and particularly to a Doppler frequency detector for generating a signal for switching equalizers each of which equalizes a fast Fourier-transformed signal, according to a Doppler frequency.
FIG. 2 is a schematic block diagram of a conventional digital terrestrial broadcasting receiver.
The digital terrestrial broadcasting receiver has an antenna 1 which receives an OFDM-modulated wireless or radiofrequency signal RE lying in an UHF (Ultra High Frequency) band, and a tuner 3 which frequency-converts the radiofrequency signal RF received by the antenna 1 in accordance with a local oscillation signal LO of a local oscillator 2 to generate or produce an intermediate frequency signal IF of a desired reception channel. The intermediate frequency signal IF is amplified in such a manner that average power becomes a constant value by a variable gain amplifier (hereinafter called “AMP”) 4 whose amplification factor or gain is controlled by a gain control signal AGC, followed by being supplied to an analog-digital converter (hereinafter called “ADC”) 5. A power calculating unit 6 is connected to an output side of the ADC 5.
The power calculating unit 6 calculates a value corresponding to the average power of the output signal of the AMP 4, based on a time domain signal converted to a digital value by the ADC 5. An output signal CON of the power calculating unit is supplied to a digital-analog converter (hereinafter called “DAC”) 7, where it is converted into an analog signal, which in turn is supplied to the AMP 4 as a gain control signal AGC.
Further, a fast Fourier transformer (hereinafter called “FFT”) 8 is connected to the output side of the ADC 5. The FFT 8 converts the time domain signal converted to the digital value by the ADC 5 to signals in a frequency domain corresponding to a plurality of carriers that constitute OFDM. An equalizer (EQU) 9, which makes synchronization between the carriers to generate received data, is connected to an output side of the FFT 8. Further, an error correction unit, a video-audio reproduction unit and the like, although not shown in the figure, are connected to an output side of the equalizer 9.
In the digital terrestrial broadcasting receiver, a desired signal is selected by the tuner 3 from the radiofrequency signal RF received by the antenna 1 and converted to an intermediate frequency signal IF. A plurality of carriers have been multiplexed into the intermediate frequency signal IF. The respective carriers have been quadrature-modulated based on data constituting broadcast contents and control signals. The intermediate frequency signal IF is amplified to a predetermined level by the AMP 4, which in turn is supplied to the ADC 5, where it is converted into a digital value in accordance with a sampling clock CLK. The received signal converted to the digital value by the ADC 5 is supplied to the FFT 8 where it is separated into signals set every carrier, thereby producing received data. The equalizer 9 makes synchronization between the carriers to produce the received data. The received data is played back or reproduced as video and voice by the video-audio reproduction unit or the like.
On the other hand, the received signal converted to the digital value by the ADC 5 is supplied to the power calculating unit 6, where its average power of a predetermined period is calculated. The value of the average power calculated by the power calculating unit 6 is converted into an analog gain control signal AGC by the DAC 7, which is then supplied to the AMP 4. At the AMP 4, its amplification factor or gain is reduced as the gain control signal AGC becomes larger, whereas the amplification factor increases as the gain control signal AGC becomes smaller. Consequently, the average power of the output signal of the AMP 4 converges to a predetermined value.
Thus, the received signal converted to the digital value is obtained suitably from the ADC 5 by setting the average power outputted from the AMP 4 so as to be the optimum input level of the ADC 5.
The above prior art refers to patent documents 1 (Japanese Unexamined Patent Publication No. 2000-22661), 2 (Japanese Unexamined Patent Publication No. 2005-45664) and 3 (Japanese Unexamined Patent Publication No. 2004-153811).
However, although the digital terrestrial broadcasting receiver is configured so as to suppress a variation in reception level and hold a satisfactory receiving state, a variation in delay time due to the Doppler phenomenon occurs along with high-speed fading at which the reception level varies with high-speed movement of a portable terminal, where the digital terrestrial broadcasting receiver is built in the portable terminal, for example. A problem arises in that there is a need to provide two or more equalizers different in characteristic and switch the same according to the conditions of a propagation path for the purpose of improving both characteristics with respect to such high-speed fading and Doppler phenomenon as described above.
When the Doppler frequency at low-speed movement or the like of the portable terminal, a change in gain control signal AGC is reduced. The digital terrestrial broadcasing receiver is accompanied by a problem that since the Doppler frequency is estimated based on the number of states or the like in which changes have occurred, the probability that an erroneous Doppler frequency will be indicated due to degradation of estimated accuracy increases.